1. Field of the Invention
The present invention relates to a deep fat food fryer wherein accurate temperature control of the cooking medium during the cooking process is desirable and wherein quick response to changes in the temperature of the cooking medium is desirable.
2. Prior Art
In deep frying of certain foods, accurate temperature control is important. In cooking donuts, for instance, it is important to maintain the desired temperature of the cooking medium through the entire cooking time.
In the past, simple mechanical on and off thermal controllers have been used to control the temperature of the cooking medium such as cooking oil. They would turn off when the cooking medium reached a certain temperature and turn back on when the temperature dropped below a given, slightly lower temperature. Hence, the temperature was never stable. Fluctuations of 10.degree. or more were not uncommon. To improve on this situation, proportioning controllers were introduced.
Proportioning controllers vary the amount of power reaching the heating element by turning the heating element on and then off periodically. The ratio of "on" time to the total time or period is known as the duty cycle. By adjusting the duty cycle or percentage of "on" time, the amount of heat generated by the heating element can be controlled. For example, if 6% heat is desired, the heater could be turned on for 6 seconds and off for 94 seconds. The period would be 100 seconds. By adjusting the percentage of "on" time, the power received by the heating element can then be continuously adjusted.
Existing proportioning type thermal controllers give precise temperature control if the load is steady. The proportioning band will be set so that if the temperature is below the band, the heater is on all the time. Conversely, if the temperature is above the proportioning band, the heater is off. If the temperature is within the band, the proportioning mechanism is activated and the following formula is used to set the heater duty cycle. ##EQU1##
It will be appreciated that as the actual temperature drops, the duty cycle will increase. Accordingly, a slight drop in temperature causes a slight increase in heating. Eventually, an equilibrium will be reached and the temperature will change very little. This assumes that the lag time is short. In other words, the period is short compared to the time it takes additional heat from the heating element to increase the actual temperature reading.
If the proportioning band width is too narrow (a small number of degrees), the actual temperature will overshoot and continue to oscillate around the proportioning band. Conversely, if the proportioning band width is too wide (a large number of degrees), the actual temperature will be allowed to fluctuate widely with load within the band. Hence, there is a natural limit to how closely a proportioning controller can hold the actual temperature to the desired set point.
Applicant is aware of the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,626,840 Day 1971 4,549,527 Davis 1985 4,812,625 Mario Ceste, Sr. 1989 4,663,710 Waugh et al. 1987 4,278,872 Koether et al. 1981 ______________________________________
Accordingly, it is a principal object and purpose of the present invention to provide a temperature control system for a deep fat fryer that will accurately control the temperature of the cooking medium during the cooking process and will quickly respond to changes in the temperature of the cooking medium.